Science of Nuclear Capacity Fuels Doubt on Deal's Impact

GENEVA—The big question over the agreement forged Sunday in this Swiss lakeside city between Iran and the world's big powers is this: How much will it delay, if at all, Tehran's ability to produce enough material for a nuclear weapon?

Israel argues that as long as Iran can enrich uranium at even a low level, it can elude detection of any clandestine effort toward making higher-level, weapons-grade fuel, and can quickly restore the...